It is frequently desirable to keep formulation components separate prior to use, for example because the components may be too unstable for long-term storage if combined. It is desirable in such cases to be able to mix the formulation components at the point of use in an efficient and simple way.
One example of a formulation where it may be desirable to keep formulation components separate is tooth-whitening formulations comprising reactive ingredients such as peroxides or peroxyacids or their precursors. For example, one may want to combine A+B or A+B+C to obtain an unstable bleach X, but keep A and B separate up to that point. The difficulty arises in that during use the mixing must be rapid, and diffusion of the bleaching agent, X, to the tooth surface must be efficient. Unfortunately, combining multiple gels or other moderately viscous materials is not generally an efficient way to quickly mix chemicals; if a typical consumer were to mix by hand, it would lead to regions of well-mixed and poorly-mixed sample. One has only to hand-mix two viscous house paints together to easily see the problem: rather than efficient blending of the two colors, laminar flow causes the colors to exist in adjacent streaks. To overcome this problem directly would require more time and mixing effort than the typical user would be willing to devote to the task, and where the reactive species X begins to break down within minutes, such a method would be unworkable.
There is thus a need for products that permit ingredients to be efficiently and effectively combined at the point of use.